The invention relates to a process for plating pin grid arrays (PGAs) and a receiving jig for PGAs. The process according to the invention is based on the use of said receiving jig for PGAs which is so designed that PGAs to be plated can be clipped on, with it being possible to reliably make contact to all the pins, and plated uniformly in an electroplating bath.
Since it has become possible to continuously increase the integration density of semiconductor circuits, the requirements imposed on the housings which receive such circuits have also increased continuously. Thus, an allotment of 225 contacts and over on a silicon chip measuring approx. 1 cm.sup.2 to an equally large number of contact pins is now no longer a rarity. It is possible to lay the conductor track from the edge of the silicon chip to a previously determined contact pin on the housing, for example, using the so-called multilayer technique in which the conductor tracks are laid above each other in several levels, possibly with interconnections. Owing to the superior material properties of ceramics, in particular Al.sub.2 O.sub.3 ceramics, such pin grid arrays are nowadays manufactured predominantly from this material. The manufacture of said PGAs is expensive since it comprises several sequentially arranged operational steps. A procedure, described as the "cofired technique", for producing multilayer ceramic housings is, for example, described in U.S. Pat. No. 3,518,756.
In particular, in the case of PGAs, on which very high requirements are imposed in subsequent use, it is not possible to dispense with gold-plating the visible, exposed conductor tracks and the connecting pins. This plating is carried out chemically or, preferably, by electrodeposition. A considerable hazard in this process step (usually the last one) in manufacturing a PGA is that correct electrical contact is not made to individual pins or parts of conductor tracks and they are consequently not coated with the required gold layer in the electroplating bath. Subsequent plating is very complicated in terms of process engineering and is, as a rule, too expensive, with the result that such incompletely plate PGAs (even when only a single pin is involved) have to be regarded as reject. However, compared with rejects in preceding process steps, a reject in the final process step means, on the other hand, a disproportionately high increase in the price of the products.
For the electrodeposition of gold or other metals from corresponding metal salt baths, an electrical contact is essential between all the areas to be plated and the cathode.
According to the prior art, this contact is made via one or more short-circuit rings which are provided in one or more layers of the multilayer PGA and connect all the printed conductor tracks of said layer to each other. These short-circuit rings are brought to the outside where electrical contact can then be made to them. A plurality of PGAs to which contact is made in this way is then clamped to an electroplating rack, an electrical contact being made to the rack which is directly connected in turn to the cathode, and the electrodeposition of metal then takes place in an electroplating bath.
The short-circuit ring has to be removed after completing the electrodeposition. This may be carried out, for example, by sawing or nicking beforehand and breaking. The subsequent removal of the short-circuit ring by breaking up the fired ceramic, for example, by sawing, lazer cutting, water-jet cutting or the like is very complicated and expensive. Although nicking in the unfired state and breaking off the short-circuit rink after firing and gold-plating is cheaper, it results in imprecise edges of fracture and chipping or the like owing to the brittle properties of the ceramic and consequently in rejects. Any subsequent working of the edge is again very cost-intensive.
Furthermore, after the short-circuit ring has been removed, the emerging metallic connection is always visible at the side edge of the PGA. In the subsequent processing steps such as mounting the semiconductor chip, further electroplating processes or subsequent use of the finished electronic component, this may result in short-circuits.
The area in each layer to be metallized increases owing to the connection of every individual conductor track outwards to the short-circuit ring. This affects the electrical characteristic data, for example the capacitances. In the housings, however, the smallest possible capacitances between the individual layers and between the conductor tracks is desirable in order to keep any mutual interaction of the signal transmission as small as possible.
In the construction of parts having a short-circuit ring, the areal proportion of the short-circuit ring occupies 20-50% of the area of the housing, depending on type and design. The additional area has to be carried through all the operational steps from the beginning to the gold-plating, and the production capacity of all the operational steps, for example printing of the conductor tracks, firing of the housings or electrodeposition of metal is reduced by this percentage or the costs are increased.
For the abovementioned reasons, the objects arose of finding a process for plating PGAs which does not employ a short-circuit ring in the PGA and consequently eliminates the abovementioned disadvantages.
This is, for example, achieved by making electrical contact via the pins (contact pins) which are all short-circuited. In this case a short-circuit ring is no longer necessary in the PGA.
U.S. Pat. No. 4,032,414 describes a jig for plating two-row pinstrips (for example, dual-in-line housing). In this case, the pinstrips are mechanically clamped onto an electrically conducting terminal bar and then conveyed by means of a conveyor belt through various electroplating baths and finally pulled off the electrically conducting bar again, also mechanically. Even with the two-row pinstrips described, however, there is the risk that, if one or more pins are bent outwards before clipping onto the terminal bar, there is no guarantee of electrical contact via all the pins. An attempt is made in a further development of the invention to counteract this disadvantage by pressing the double row of pins not only onto one terminal bar but by additionally pressing the two rows of pins from the outside with two further bars onto the terminal bar situated on the inside. This requires, however, a further jig with a spring mechanism which has to be opened before the pinstrips are clipped on and has to be closed after the pinstrips have been positioned on the inner terminal bar, the two rows of pins being pressed against the inner terminal bar. The inner terminal bay may also be wedge-shaped.
This process is unsuitable for plating a pin grid arrays since it is either impossible to ensure a reliable contact to all the pins (and this is a considerable disadvantage in the case of the substantially more expensive multilayer PGAs) or the jig which makes reliable contacting possible is constructionally too complicated and consequently makes the entire process uneconomical.